Method of controlling urea dosing in an exhaust system of a vehicle

ABSTRACT

The present application relates to a method of controlling urea dosing in an exhaust system of a vehicle. The method includes adjusting the urea dosing based on the severity of urea deposition on an SCR catalyst, where the urea dosing is reduced as the severity increases.

RELATED APPLICATIONS

This application claims priority to European Patent Application Serial No. EP10159228.5, filed on Apr. 7, 2010, the entire contents of which being incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method of controlling urea dosing in an exhaust system of a vehicle, wherein said urea is injected into an exhaust gas flow in said exhaust system for selective catalytic reduction.

BACKGROUND AND SUMMARY

It is well known to use urea-water solutions as reductants injected into automotive exhaust systems in order to react over a selective catalytic reduction catalyst (SCR catalyst) with nitrogen oxides (NO_(x)) to decrease automotive NO_(x) emissions. Specifically, urea solution is injected into the exhaust by either air-assisted or hydraulic injection hardware, wherein the amount of injection is calculated using a control strategy depending on the exhaust gas mass flow rate (EGMFR) and characteristic exhaust temperature (T_(char)), the content of nitrogen oxides in the exhaust gas or urea-water solution dosing rate (UDR), and properties of the SCR catalyst (such as conversion efficiency versus catalyst conditions including amount of stored ammonia, ageing status, etc.).

A problem arising in practice is that if the urea solution is dosed in a significant amount, which may be necessary for reaction with a significant amount of NO_(x), there is a risk or possibility of urea-derived deposit formation on surfaces of the exhaust system downstream of the urea injector. These deposits may cause an insufficient urea usage and/or a plugging of the SCR catalyst resulting in malfunctions of the exhaust systems (i.e. high backpressure clogging and increased NO_(x) emissions etc.).

It is therefore desirable to avoid or at least minimize the possibility of such negative effects by limiting an amount or rate of injection of urea solution that may cause urea-derived deposit formation via a control system during vehicle operation, and to provide effective methods to avoid urea-derived deposit formation.

WO 2009/112129 A1 discloses a metering system for injecting a urea solution into an exhaust gas flow of an internal combustion engine for selective catalytic reduction and a method for controlling the injecting of a urea solution by means of compressed air. The metering system can be connected to a urea solution tank, wherefrom urea solution can be removed. The metering system comprises at least one nozzle through which the urea solution can be injected into the exhaust gas flow by means of compressed air. The metering system also comprises an air valve by means of which the pressure and/or the quantity of air and/or the valve opening times of the compressed air supply can be regulated. A sensor for measuring the pressure and/or the quantity of air is disposed in the compressed air supply between the air valve and the nozzle, so that the quantity of compressed air fed in for atomizing the urea solution is controlled and lowered to the minimum air quantity required at a given moment, depending on the operating parameters of exhaust gas temperature and exhaust gas mass flow. However, in this system, only a minimum amount of urea solution is atomized by the compressed air at any given operating condition of the vehicle. Further, control of the rate or amount of urea injected into the exhaust gas flow is based on exhaust gas temperature, exhaust gas mass flow, urea mass stream, required degree of efficiency of the catalyst, catalyst size, and size of a preparation section between the metering system and the catalyst, and thus calculation of a desired urea injection or dosing rate is complicated.

The inventors herein have recognized the above problems and have devised an approach to at least partially address them. The present invention provides a method of controlling the urea dosing in an exhaust system of a vehicle, wherein urea-derived deposit formation may be at least substantially reduced by alternately operating the exhaust system under more than one dosing function according to an allocation rule. As such, a first reductant dosing mode comprises controlling urea dosing as a function of T_(char), EGMFR, NO_(x) emissions, stored NH₃ supply, NH₃ desired, engine speed/torque, allowable NH₃ slip, etc. A second reductant dosing mode comprises restricting urea dosing based on a mapped deposition-related characteristic, wherein a urea deposition severity index (UDSI) value is calculated, and if the UDSI value is above a threshold, according to the allocation rule, the rate and/or amount of urea solution injected into the exhaust gas flow is decreased until the UDSI is below the threshold. If the UDSI value is below the threshold, according to the allocation rule, the exhaust system will continue or will revert to operating in the first reductant dosing mode.

In one specific example, a method for controlling urea dosing in an exhaust system of a vehicle, wherein said urea is injected into an exhaust gas flow in said exhaust system for selective catalytic reduction, comprises the following steps: establishing an allocation rule for a group of parameters that characterize an operating condition in said exhaust system, and for a value of an UDSI suited to characterize an urea deposition severity in said exhaust system; determining a current value combination of said group of parameters for a current operation condition of said exhaust system; based on said allocation rule, allocating a current value combination for said group of parameters for a current operating condition of said exhaust system, to a current value of said UDSI; and controlling a urea dosing quantity in said exhaust system based on said current value of said UDSI.

Thus, the present invention is, inter alia, based on the concept to monitor the severity of the urea-derived deposit formation based on an appropriate UDSI against exhaust gas conditions and exhaust system geometry, and uses this index for correcting (i.e. restricting, if necessary) the urea dosing (i.e. the injected urea solution amount) such that formation of the urea-originated deposits on inner surfaces of the exhaust system components, especially between the urea injector and inlet into SCR catalyst, is reduced or even avoided.

It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of an engine including an example exhaust treatment system.

FIG. 2 shows a flow chart illustrating a control routine for controlling a urea dosing rate (UDR).

FIG. 3 shows an example map of empirical data.

FIG. 4 shows an example map of empirical data including designated urea severity deposit index (UDSI) values.

DETAILED DESCRIPTION

The following description relates to a method for controlling a diesel aftertreatment system, which includes a selective catalytic reduction (SCR) catalyst and an injector for injecting reductant into an exhaust gas flow path coupled to an exhaust system in an engine of a motor vehicle. FIG. 1 shows an example engine wherein the present method may be employed. The method is carried out via a controller, the controller including instructions for an allocation rule, and being in communication with the various sensors and actuators of the motor vehicle.

During operation of the vehicle, a first reductant dosing mode comprises controlling urea dosing as a function of characteristic exhaust temperature (T_(char)), exhaust gas mass flow rate (EGMFR), NO_(x) emissions, stored NH₃ supply, NH₃ desired, engine speed/torque, allowable NH₃ slip, etc. Further, the controller monitors a ratio of the EGMFR to a urea-water solution mass flow rate or dosing rate (UDR), and the T_(char). The controller references a predetermined urea deposition severity index (UDSI) map to compare a ratio of UDR/EGMFR and the T_(char) to determine a UDSI value. If the UDSI value is greater than a threshold, a second reductant dosing mode is initiated and the first dosing method is stopped. In the secondary reductant dosing mode, the amount/rate of injected reductant is decreased. If the UDSI value is less than the threshold, the primary reductant dosing method is continued.

In this manner, when deposition of urea on the SCR catalyst is likely to occur, the amount of urea injected into the exhaust gas flow path is decreased such that urea deposits are minimized or avoided. Thus, the present method increases the life and efficiency of the SCR catalyst. This method is illustrated in the flow chart shown in FIG. 2. Further, FIG. 3 includes a diagram of the results of an exemplarily series of measurements (data array) plotted according to the present invention, and FIG. 4 illustrates a possible allocation rule that is adapted to the diagram of FIG. 3 according to an embodiment of the present invention.

FIG. 1 is a schematic diagram showing one cylinder of multi-cylinder engine 10, which may be included in a propulsion system of an automobile. Engine 10 may be controlled at least partially by a control system including controller 12 and by input from a vehicle operator 132 via an input device 130. In this example, input device 130 includes an accelerator pedal and a pedal position sensor 134 for generating a proportional pedal position signal (PP). Combustion chamber (i.e., cylinder) 30 of engine 10 may include combustion chamber walls 32 with piston 36 positioned therein. Piston 36 may be coupled to crankshaft 40 so that reciprocating motion of the piston is translated into rotational motion of the crankshaft. Crankshaft 40 may be coupled to at least one drive wheel of a vehicle via an intermediate transmission system. Further, a starter motor may be coupled to crankshaft 40 via a flywheel to enable a starting operation of engine 10.

Combustion chamber 30 may receive intake air from intake manifold 44 via intake passage 42 and may exhaust combustion gases via exhaust passage 48. Intake manifold 44 and exhaust passage 48 can selectively communicate with combustion chamber 30 via respective intake valve 52 and exhaust valve 54. In some embodiments, combustion chamber 30 may include two or more intake valves and/or two or more exhaust valves.

In this example, intake valve 52 and exhaust valves 54 may be controlled by cam actuation via respective cam actuation systems 51 and 53. Cam actuation systems 51 and 53 may each include one or more cams and may utilize one or more of cam profile switching (CPS), variable cam timing (VCT), variable valve timing (VVT) and/or variable valve lift (VVL) systems that may be operated by controller 12 to vary valve operation. The position of intake valve 52 and exhaust valve 54 may be determined by position sensors 55 and 57, respectively. In alternative embodiments, intake valve 52 and/or exhaust valve 54 may be controlled by electric valve actuation. For example, cylinder 30 may alternatively include an intake valve controlled via electric valve actuation and an exhaust valve controlled via cam actuation including CPS and/or VCT systems.

Fuel injector 66 is shown coupled directly to combustion chamber 30 for injecting fuel directly therein. Fuel injection may be via a common rail system, or other such diesel fuel injection system. Fuel may be delivered to fuel injector 66 by a high pressure fuel system (not shown) including a fuel tank, a fuel pump, and a fuel rail.

Intake passage 42 may include a throttle 62 having a throttle plate 64. In this particular example, the position of throttle plate 64 may be varied by controller 12 via a signal provided to an electric motor or actuator included with throttle 62, a configuration that is commonly referred to as electronic throttle control (ETC). In this manner, throttle 62 may be operated to vary the intake air provided to combustion chamber 30 among other engine cylinders. The position of throttle plate 64 may be provided to controller 12 by throttle position signal TP. Intake passage 42 may include a mass air flow sensor 120 and a manifold air pressure sensor 122 for providing respective signals MAF and MAP to controller 12.

Further, in the disclosed embodiments, an exhaust gas recirculation (EGR) system may route a desired portion of exhaust gas from exhaust passage 48 to intake passage 44 via EGR passage 140. The amount of EGR provided to intake passage 44 may be varied by controller 12 via EGR valve 142. Further, an EGR sensor 144 may be arranged within the EGR passage and may provide an indication of one or more pressure, temperature, and concentration of the exhaust gas. Alternatively, the EGR may be controlled through a calculated value based on signals from the MAF sensor (upstream), MAP (intake manifold), MAT (manifold gas temperature) and the crank speed sensor. Further, the EGR may be controlled based on an exhaust O₂ sensor and/or an intake oxygen sensor (intake manifold). Under some conditions, the EGR system may be used to regulate the temperature of the air and fuel mixture within the combustion chamber. While FIG. 1 shows a high pressure EGR system, additionally, or alternatively, a low pressure EGR system may be used where EGR is routed from downstream of a turbine of a turbocharger to upstream of a compressor of the turbocharger.

As such, engine 10 may further include a compression device such as a turbocharger or supercharger including at least a compressor 162 arranged along intake manifold 44. For a turbocharger, compressor 162 may be at least partially driven by a turbine 164 (e.g. via a shaft) arranged along exhaust passage 48. For a supercharger, compressor 162 may be at least partially driven by the engine and/or an electric machine, and may not include a turbine. Thus, the amount of compression provided to one or more cylinders of the engine via a turbocharger or supercharger may be varied by controller 12.

Exhaust gas sensor 126 is shown coupled to exhaust passage 48 upstream of emission control system 70. Sensor 126 may be any suitable sensor for providing an indication of exhaust gas air/fuel ratio such as a linear oxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), a two-state oxygen sensor or EGO, a HEGO (heated EGO), a NO_(x), HC, or CO sensor. Further, an exhaust gas temperature sensor 128 is included at the same location, and reports the T_(char) between the injector and the SCR catalyst. In an alternate embodiment, the exhaust temperature sensor and the exhaust gas sensor may be disposed in the exhaust passage 48 at different locations.

Emission control system 70 is shown arranged along exhaust passage 48 downstream of sensors 126 and 128. System 70 may be a selective catalytic reduction (SCR) system, a three way catalyst (TWC), NO_(x) trap, various other emission control devices, or combinations thereof. For example, device 70 may be an exhaust aftertreatment system which includes an SCR catalyst 71 and a diesel particulate filter (DPF) 72. In some embodiments, DPF 72 may be located downstream of the catalyst (as shown in FIG. 1), while in other embodiments, DPF 72 may be positioned upstream of the catalyst (not shown in FIG. 1). The DPF may be thermally regenerated periodically during engine operation. Further, in some embodiments, during operation of engine 10, emission control system 70 may be periodically reset by operating at least one cylinder of the engine within a particular air/fuel ratio.

In one example, a urea injection system 80 is provided to inject liquid urea to SCR catalyst 71. The urea injection system 80 includes an injector 82, which is configured to inject a liquid reductant, such as a urea solution, into an exhaust gas flow path within exhaust passage 48. In the present embodiment, the injector 82 is angled relative to the exhaust passage 48. In alternate embodiments the injector may be either of parallel to or perpendicular to the exhaust passage. Further, the injector may include either air-assisted or hydraulic injection hardware.

Controller 12 is shown in FIG. 1 as a microcomputer, including microprocessor unit 102, input/output ports 104, an electronic storage medium for executable programs and calibration values shown as read only memory chip 106 in this particular example, random access memory 108, keep alive memory 110, and a data bus. Controller 12 may receive various signals from sensors coupled to engine 10, in addition to those signals previously discussed, including measurement of inducted mass air flow (MAF) from mass air flow sensor 120; engine coolant temperature (ECT) from temperature sensor 112 coupled to cooling sleeve 114; a profile ignition pickup signal (PIP) from Hall effect sensor 118 (or other type) coupled to crankshaft 40; throttle position (TP) from a throttle position sensor; and absolute manifold pressure signal, MAP, from sensor 122. Engine speed signal, RPM, may be generated by controller 12 from signal PIP. Manifold pressure signal MAP from a manifold pressure sensor may be used to provide an indication of vacuum, or pressure, in the intake manifold. In one example, sensor 118, which is also used as an engine speed sensor, may produce a predetermined number of equally spaced pulses every revolution of the crankshaft.

Storage medium read-only memory 106 can be programmed with computer readable data representing instructions executable by processor 102 for performing the described methods as well as other variants that are anticipated but not specifically listed.

As described above, FIG. 1 shows only one cylinder of a multi-cylinder engine, and that each cylinder may similarly include its own set of intake/exhaust valves, fuel injector, spark plug, etc.

An example method 200 for operating the exhaust system depicted in FIG. 1 is shown in FIG. 2. The controller 12 may carry out the method 200 via communication with various sensors and actuators in the engine and exhaust system. The method 200 comprises an “allocation rule” for determining an amount of reductant dosing for limiting deposits on the catalyst.

In 210, the controller calculates a ratio between the current UDR and the current EGMFR (UDR/EGMFR). Further, the controller determines the T_(char) via the exhaust temperature sensor 128. Then, in 212, the controller determines a UDSI value by referencing a predetermined map of the UDR/EGMFR ratio vs. T_(char). An example of predetermined map is shown in FIG. 4 (described later).

In 214, the controller determines if the UDSI is greater than a threshold. An example of the threshold, according to the example shown in FIG. 4, is 3. If the UDSI is less than the threshold, at 216 and 218, the controller commands the exhaust system to operate in a first reductant dosing mode, comprising a reductant dosing rate determined based on current operating conditions of the vehicle. The current operating conditions may include one or more of T_(char), EGMFR, NO_(x) emissions, stored NH₃ supply, NH₃ desired, engine speed/torque, allowable NH₃ slip, properties of the SCR, etc.

However, if the controller determines that the UDSI is greater than the threshold, at 220, the controller commands the exhaust system to operate in a second reductant dosing mode. As shown in 222, the second reductant dosing mode comprises a reduced/restricted urea dosing mode. The exhaust system is operated in the second reductant dosing mode until the UDSI is reduced to a value below the threshold. Thus, the second reductant dosing mode is based on the T_(char) and the ratio between the UDR and the EGMFR. The UDSI may continuously be determined by the controller in order to regulate reductant dosing. Alternatively, the UDSI may be periodically determined by the controller in order to regulate reductant dosing.

According to this embodiment, the group of parameters for the second reductant dosing mode comprises only two parameters, wherein a first parameter depends on the UDR/EGMFR ratio, and the other parameter is the T_(char) of the exhaust system. According to an alternate embodiment, the group of parameters for the second reductant dosing mode comprises one parameter, which is the UDR/EGMFR ratio. In each of these embodiments, the present invention involves the concept of combining two variables, UDR and EGMFR, into a new variable UDR/EGMFR, which allows the UDSI to be mapped against any of the principal parameters of the system, such as exhaust gas mass flow rate, urea dosing rate, and characteristic temperature, using only one map. This concept is based on the consideration that a direct evaluation or usage of the measured variables in controlling or avoiding deposits formation would require a number of maps (i.e. not only one map) for different dosing rates which would not be convenient to be used in controlling the urea dosing in the exhaust system. In contrast, as a result of the present invention, the obtained map can be used by the control strategy as a map or “look-up table” of the UDSI value versus temperature (i.e., T_(char) upstream of SCR catalyst) and the above mentioned combined variable (the UDR/EGMFR ratio).

In order to carry out the method and allocation rule described above, the map and look-up table of urea deposition severity may be empirically determined via a urea deposition severity analysis. The UDSI, being suited to characterize a urea deposition severity in the exhaust system, is defined based on the percentage of the deposits initially formed during urea dosing that is not removed within a predetermined period after reducing urea dosing.

In other words, based upon the urea deposition severity analysis, a method of controlling urea dosing in an exhaust system according to the invention may result in a maximum allowed dosing rate of the urea solution with which the formation of the urea-derived deposits in the exhaust system is either avoided, and/or minimized so that even if formed, these deposits can be easily decomposed in the exhaust. In the urea deposition severity analysis, among the physical factors affecting deposit formation, the main factors are assumed to be the EGMFR, the UDR, the exhaust geometry in between the urea injector and the SCR catalyst, and the T_(char), in particular between the urea injector and the SCR catalyst. These factors may be taken into account, and are, according to the present invention, combined in a simple way into new variable or map that can be relatively simply implemented within the control strategy of the operation of the urea SCR system.

Specifically, a urea deposition severity analysis approach according the present invention may comprise the following steps:

-   a) Developing experimental matrix that preferably covers a wide     range of changing of the above-mentioned parameters (i.e. EGMFR,     UDR, T_(char)) -   b) Developing a hardware representative for the application of     interest (engine with the exhaust system etc.) -   c) Running deposition experiments for the developed matrix -   d) Evaluating deposition either directly against measured variables,     or against new variables obtained by combining measured variables     into new dimension or dimensionless variables -   e) Constructing 2D-table (map) containing deposition-related     characteristic plotted against two key directly measured or new     combined variables.     The constructed 2D-table or map can be easily used within the     control strategy and allows restricting of the UDR based on the     mapped deposition-related characteristic and request for the UDR     coming from the SCR system control modules.

FIG. 3 is a map of the results of an exemplarily series of measurements (data array) plotted according to the present invention, and FIG. 4 illustrates a possible allocation rule that is adapted to the diagram of FIG. 1 according to an embodiment of the present invention.

First, in order to establish an allocation rule, a series of measurements were performed by developing an experimental matrix covering a wide range of changing of the parameters including the EGMFR, the UDR, and the T_(char) by running deposition experiments for the developed matrix in an engine with an exhaust system. As temperature (T_(char)), the exhaust gas temperature upstream of the SCR catalyst was used.

It was found that for T_(char) above 350° C., no urea-originated deposits were found in the exhaust system (within whole ranges of exhaust gas mass flow) irrespective of the urea dosing rate up to the maximal dosing rate according to the urea injector design. Furthermore, within the control system for SCR technology, a low-temperature limit for the operation for SCR system exists. This limit was also taken into account during development of the experimental conditions matrix. However, experimental conditions were extended beyond this low-temperature limit, down to values for T_(char) of approximately 150° C.

For a complete experimental matrix the following ranges for key parameters have been chosen: EGMFR of 59-369 kg/h; UDR of 0-304 mg/s; T_(char) of 150-400° C. The UDSI value can be defined, just as an example and without limitation of the invention, as the percentage of the deposits initially formed during urea dosing that is not removed within 5 minutes in given conditions after stopping urea dosing. More specifically, quantitative values can be given as defined in Table 1:

TABLE 1 UDSI-value Criterion 0 Deposits were not observed, or 100% of the initially formed deposits were removed in 5 minutes after stopping urea dosing 1 0-25% of the initially formed deposits were not removed in 5 min after stopping urea dosing 2 25-50% of the initially formed deposits were not removed in 5 min after stopping urea dosing 3 50-75% of the initially formed deposits were not removed in 5 min after stopping urea dosing 4 75-100% of the initially formed deposits were not removed in 5 min after stopping urea dosing

An example of a general procedure for the experiments is as follows, for engine speeds from 3.000 rpm down to 1.000 rpm (in steps of 500 rpm), characteristic temperatures T_(char)=400° C. down to 150° C. (in steps of 50° C.), with stabilized engine and exhaust parameters and for the values of dosing rates 0, 5, 10, 20, 50, 100, 200 and 300 mg/s, the following steps were performed: switch dosing on, keep on dosing to observe dosing-on features, turn dosing off, observe dosing-off features, end the current dosing rate, end the current T_(char)-point, and end the current engine speed.

For example, one of the conducted experiments was started for an engine speed of 3.000 rpm and for an engine load sufficient to provide a characteristic temperature (e.g. the exhaust temperature upstream of the SCR catalyst) of 400° C. Then, in this engine operating (speed/load) point, different urea dosing rates were applied (i.e. urea dosing was first switched on, kept for period of time sufficient to observe features, and then switched back off) and the status of the deposition was monitored (in terms of rate of appearance/disappearance, reversibility, stability etc.) from low to high urea dosing rates.

After performing this experiment, the quartz tube was washed (if necessary) and dried, and then the engine load was changed to a lower value in order to provide a characteristic temperature which was 50° C. lower. After achieving the minimal scheduled T_(char) of roughly 150° C., the same sequence of experiments was carried out at the next engine speed, and so on.

Table 2 shows the results of an exemplary series of measurements which were performed and analyzed as explained above.

TABLE 2 Meas. Engine Urea Dosing Point Speed T_(upstream SCR) rate UDSI No. [RPM] [° C.] [mg/sec] value  1 1000 260  6.8 0  2 1000 234  6.8 1  3 1000 204  6.8 1  4 1000 178  6.8 2  5 2000 390  6.8 0  6 2000 350  6.8 0  7 2000 302  6.8 0  8 2000 252  6.8 0  9 2000 206  6.8 1 10 2000 173  6.8 1 11 2000 154  6.8 2 12 2500 344  6.8 0 13 2500 304  6.8 0 14 2500 253  6.8 0 15 2500 204  6.8 1 16 2500 176  6.8 1 17 2500 154  6.8 2 18 3000 351  6.8 0 19 3000 304  6.8 0 20 3000 255  6.8 0 21 3000 209  6.8 0 22 3000 177  6.8 1 23 1500 395 10.3 0 24 1500 359 10.3 0 25 1500 310 10.3 0 26 1500 265 10.3 0 27 1500 174 10.3 1 28 1500 148 10.3 3

These results are used according to the invention in a dosing strategy to control (in particular to at least substantially reduce) the urea dosing of an exhaust system of a vehicle. To this, the two variables, the UDR and the EGMFR, were combined in a new variable defined as the UDR/EGMFR ratio, which allows for the UDSI (defined as described above) to be mapped against any principal parameter of the system, such as exhaust gas mass flow rate, urea dosing rate, and characteristic temperature using only one map as illustrated in FIG. 3.

In FIG. 4, five different regions have been separated in this 2D-map depending on the respective value of the UDSI (i.e. values from UDSI 0-4). A possible use of the map according to the present invention in a control strategy can be as follows: operating conditions, in which probability of the deposit formation and not removal is high (e.g. if more than 50% of the initially formed deposit retains after 5 minutes) may be those which are characterized by the UDSI value of 3 and 4. If the controller detects/determines that UDSI has a value of 3 or higher, it will restrict the allowed amount of the urea solution down to a value for which the UDSI will be equal or less than 2.

The above description of preferred embodiments has been given by way of example. A person skilled in the art will, however, not only understand the present invention and its advantages, but will also find suitable modifications thereof. Therefore, the present invention is intended to cover all such changes and modifications as far as falling within the spirit and scope of the invention as defined in the appended claims and the equivalents thereof. 

1. A method of controlling urea dosing in an exhaust system of a vehicle wherein said urea is injected into an exhaust gas flow in said exhaust system for selective catalytic reduction, said method comprising: establishing an allocation rule for a group of parameters that characterize an operating condition in said exhaust system, and for a value of a urea deposition severity index suited to characterize an urea deposition severity in said exhaust system; determining a current value combination for said group of parameters for a current operation condition of said exhaust system; based on said allocation rule, allocating a current value combination for said group of parameters for a current operation condition of said exhaust system, to a current value of said urea deposition severity index; and based on said current value of said urea deposition severity index, controlling urea dosing in said exhaust system.
 2. The method according to claim 1, wherein said group of parameters comprises an exhaust gas mass flow rate, a urea dosing rate, and a characteristic temperature in said exhaust system.
 3. The method according to claim 1, wherein said group of parameters comprises one parameter dependent on a ratio between the urea dosing rate and the exhaust gas mass flow rate.
 4. The method according to claim 1, wherein said group of parameters comprises two parameters, a first parameter being dependent on a ratio between a urea dosing rate and an exhaust gas mass flow rate, and a second parameter being a characteristic temperature in said exhaust system.
 5. The method according to claim 4, wherein said exhaust system comprises a selective catalytic reduction catalyst, wherein said characteristic temperature is a temperature of exhaust gas upstream of the selective catalytic reduction catalyst.
 6. The method according to claim 1, wherein said urea deposition severity index suited to characterize an urea deposition severity in said exhaust system is defined based on a percentage of deposits initially formed during urea dosing that are not removed within a predetermined period after stopping urea dosing.
 7. The method according to claim 1, wherein said allocation rule is established as a look-up table.
 8. The method according to claim 1, wherein controlling urea dosing in said exhaust system comprises reducing a urea dosing rate if said current value of said urea deposition severity index exceeds a predetermined threshold value.
 9. A system for controlling urea dosing in an exhaust system of a vehicle, comprising a controller with a chip including non-transitory computer instructions to operate a urea dosing system in a first reductant dosing mode when a urea deposition severity index is less than a threshold, the first reductant dosing mode including adjusting reductant injection into engine exhaust to a first amount based on a plurality of operating conditions, and operate the urea dosing system in a second reductant dosing mode when the urea deposition severity index is greater than the threshold, the second reductant dosing mode being a restricted urea dosing mode including reducing the first amount to a second, lower, amount.
 10. The system of claim 9, wherein the controller with the chip further includes non-transitory computer instructions for determining the urea deposition severity index.
 11. The system of claim 10, wherein determination of the urea deposition severity index comprises calculation of a ratio of an exhaust gas mass flow rate to a urea dosing rate, measurement of a characteristic exhaust temperature, and referencing a predetermined map of the ratio between the exhaust gas mass flow rate and the urea dosing rate versus the characteristic exhaust temperature.
 12. The system of claim 9, wherein the controller with the chip further includes non-transitory computer instructions for determining the first amount in the first reductant dosing mode based on one or more of the exhaust gas mass flow rate, the exhaust temperature, the urea dosing rate, a remaining amount of stored urea, an engine speed/torque, an allowable urea slip, and a plurality of properties of a SCR catalyst.
 13. The system of claim 12, wherein the plurality of properties of the SCR catalyst comprise one or more of conversion efficiency, catalyst conditions, and ageing status.
 14. A method of urea injection into an exhaust gas flow, comprising: in a first mode, dosing a first urea amount to an SCR catalyst based on a plurality of operating parameters; and in a second mode, different than the first mode, dosing a second urea amount to the SCR catalyst based on an exhaust temperature and a ratio between an exhaust gas mass flow rate and a urea dosing rate.
 15. The method of claim 14, wherein the plurality of operating parameters comprises one or more of the exhaust gas mass flow rate, the exhaust temperature, the urea dosing rate, a remaining amount of stored urea, an engine speed/torque, an allowable urea slip, and a plurality of properties of a SCR catalyst.
 16. The method of claim 15, wherein the plurality of properties of the SCR catalyst comprise one or more of conversion efficiency, catalyst conditions, and ageing status.
 17. The method of claim 14, further comprising determining a urea deposition severity index, and further adjusting urea dosing to either of the first mode or the second mode based on the urea deposition severity index.
 18. The method of claim 14, wherein determination of the urea deposition severity index comprises calculating the ratio of the exhaust gas mass flow rate to the urea dosing rate, determining the exhaust temperature, and referencing a predetermined map of the ratio between the exhaust gas mass flow rate and the urea dosing rate versus the exhaust temperature.
 19. The method of claim 18, wherein if the urea deposition severity index is greater than a threshold, the first urea amount is delivered, and if the severity index is less than the threshold, the second urea amount is delivered.
 20. The method of claim 14, wherein the second urea amount is less than the first urea amount. 